Laser powder bed fusion(LPBF)is a widely recognized additive manufacturing technology that can fabricate complex components rapidly through layer-by-layer formation.However,there is a paucity of research on the effect...Laser powder bed fusion(LPBF)is a widely recognized additive manufacturing technology that can fabricate complex components rapidly through layer-by-layer formation.However,there is a paucity of research on the effect of laser scanning speed on the cellular microstructure and mechanical properties of martensitic stainless steel.This study systematically investigated the influence of laser scanning speed on the cellular microstructure and mechanical properties of a developed Fe11Cr8Ni5Co3Mo martensitic stainless steel produced by LPBF.The results show that increasing the laser scanning speed from 400 to 1000 mm/s does not lead to a noticeable change in the phase fraction,but it reduces the average size of the cellular microstructure from 0.60 to 0.35μm.The scanning speeds of 400 and 1000 mm/s both had adverse effects on performances of sample,resulting in inadequate fusion and keyhole defects respectively.The optimal scanning speed for fabricating samples was determined to be 800 mm/s,which obtained the highest room temperature tensile strength and elongation,with the ultimate tensile strength measured at(1088.3±2.0)MPa and the elongation of(16.76±0.10)%.Furthermore,the mechanism of the evolution of surface morphology,defects,and energy input were clarified,and the relationship between cellular microstructure size and mechanical properties was also established.展开更多
Background Triglyceride glucose (TyG) index is a novel marker for metabolic disorders, and recently it has been reported to be associated with cardiovascular disease (CVD) risk in apparently healthy individuals.Howeve...Background Triglyceride glucose (TyG) index is a novel marker for metabolic disorders, and recently it has been reported to be associated with cardiovascular disease (CVD) risk in apparently healthy individuals.However the prognostic value of TyG index in patients with stable coronary artery disease (CAD) is not determined.展开更多
Time?varying mesh stiffness(TVMS) and gear errors include short?term and long?term components are the two main internal dynamic excitations for gear transmission. The coupling relationship between the two factors is u...Time?varying mesh stiffness(TVMS) and gear errors include short?term and long?term components are the two main internal dynamic excitations for gear transmission. The coupling relationship between the two factors is usually neglected in the traditional quasi-static and dynamic behaviors analysis of gear system. This paper investigates the influence of short?term and long?term components of manufacturing errors on quasi?static and dynamic behaviors of helical gear system considering the coupling relationship between TVMS and gear errors. The TVMS, loaded static transmission error(LSTE) and loaded composite mesh error(LCMS) are determined using an improved loaded tooth contact analysis(LTCA) model. Considering the structure of shaft, as well as the direction of power flow and bearing location, a precise generalized finite element dynamic model of helical gear system is developed, and the dynamic responses of the system are obtained by numerical integration method. The results suggest that lighter loading conditions result in smaller mesh stiffness and stronger vibration, and the corresponding resonance speeds of the system become lower. Long?term components of manufacturing errors lead to the appearance of sideband frequency components in frequency spectrum of dynamic responses. The sideband frequency components are predominant under light loading conditions. With the increase of output torque, the mesh frequency and its harmonics components tend to be enhanced relative to sideband frequency components. This study can provide effective reference for low noise design of gear transmission.展开更多
It has been demonstrated that pitavastatin can significantly reduce low-density lipoprotein(LDL)cholesterol(LDL-C),but its impact on lipoprotein subfractions and oxidized low-density lipoprotein(oxLDL)has not been det...It has been demonstrated that pitavastatin can significantly reduce low-density lipoprotein(LDL)cholesterol(LDL-C),but its impact on lipoprotein subfractions and oxidized low-density lipoprotein(oxLDL)has not been determined.The aim of the present study was to investigate the potential effects of pitavastatin on subfractions of LDL and high-density lipoprotein(HDL)as well as oxLDL in untreated patients with coronary atherosclerosis(AS).Thirty-six subjects were enrolled in this study.O f them,18 patients with AS were administered pitavastatin 2 mg/day for 8 weeks and 18 healthy subjects without therapy served as controls.The plasma lipid profile,lipoprotein subfractions and circulating oxLDL were determined at baseline and 8 weeks respectively.The results showed that pitavastatin treatment indeed not only decreased LDL-C,total cholesterol(TC),triglycerides(TG)and apolipoprotein B(ApoB)levels,and increased HDL cholesterol(HDL-C),but also reduced the cholesterol concentration of all of the LDL subfractions and the percentage of intermediate and small LDL subfractions.Meanwhile,pitavastatin could decrease plasma oxLDL levels.Furthermore,a more close correlation was found between oxLDL and LDL-C as well as LDL subfractions after pitavastatin treatment.We concluded that a moderate dose of pitavastatin therapy not only decreases LDL-C and oxLDL concentrations but also improves LDL subfractions in patients with AS.展开更多
Reducing the radiated noise of a gearbox is a difficult problem in aviation,navigation,machinery,and other fields.Structural improvement is the main means of noise reduction for a gearbox,and it is realized primarily ...Reducing the radiated noise of a gearbox is a difficult problem in aviation,navigation,machinery,and other fields.Structural improvement is the main means of noise reduction for a gearbox,and it is realized primarily through contribution analysis and structure optimization.However,these approaches have certain limitations.In this study,a low-noise design method for a gearbox that combines the two approaches is proposed,and experimental verification is performed.First,a finite element/boundary element model is established using a single-stage herringbone gearbox.Considering the vibration excitation of the gear system,the radiation noise of a single-stage gearbox is predicted based on the modal acoustic transfer vector(MATV)method.Subsequently,the maximum field point of the radiated noise is determined,and the acoustic transfer vector(ATV)analysis and modal acoustic contribution(MAC)analysis are conducted to determine the region that contributes significantly to the radiated noise of the field point.The optimization region is selected through the panel acoustic contribution(PAC)analysis.Next,to reduce the normal speed in the optimization region,topology optimization is performed.According to the topology optimization results,four different noise reduction structures are added to the gearbox,and the low-noise optimization models are established respectively.Finally,by measuring the radiated noise of the gearbox before and after optimization under a given working condition,the validity of the radiated noise prediction method and the low-noise optimization design method are verified by comparing the simulation and experimental data.A comparison of the four optimization models proves that the noise reduction effect can be achieved only by adding a noise reduction structure to the center of the density nephogram.展开更多
This paper describes a computational study of the hydrodynamics of a ray-inspired underwater vehicle conducted concurrently with experimental measurements. High-resolution stereo-videos of the vehicle＇s fin motions d...This paper describes a computational study of the hydrodynamics of a ray-inspired underwater vehicle conducted concurrently with experimental measurements. High-resolution stereo-videos of the vehicle＇s fin motions during steady swimming are obtained and used as a foundation for developing a high fidelity geometrical model of the oscillatory fin. A Cartesian grid based immersed boundary solver is used to examine the flow fields produced due to these complex artificial pectoral fin kinematics. Simulations are carried out at a smaller Reynolds number in order to examine the hydrodynamic performance and understand the resultant wake topology. Results show that the vehicle＇s fins experience large spanwise inflexion of the distal part as well as moderate chordwise pitching during the oscillatory motion. Most thrust force is generated by the distal part of the fin, and it is highly correlated with the spanwise inflexion. Two sets of inter-connected vortex rings are observed in the wake right behind each fin. Those vortex rings induce strong backward flow jets which are mainly responsible for the fin thrust generation.展开更多
The design synthesis is the key issue in the mechanical conceptual design to generate the design candidates that meet the design requirements.This paper devotes to propose a novel and computable synthesis approach of ...The design synthesis is the key issue in the mechanical conceptual design to generate the design candidates that meet the design requirements.This paper devotes to propose a novel and computable synthesis approach of mechanisms based on graph theory and polynomial operation.The graph framework of the synthesis approach is built firstly,and it involves:(1)the kinematic function units extracted from mechanisms;(2)the kinematic link graph that transforms the synthesis problem from mechanical domain into graph domain;(3)two graph representations,i.e.,walk representation and path representation,of design candidates;(4)a weighted matrix theorem that transforms the synthesis process into polynomial operation.Then,the formulas and algorithm to the polynomial operation are presented.Based on them,the computational flowchart to the synthesis approach is summarized.A design example is used to validate and illustrate the synthesis approach in detail.The proposed synthesis approach is not only supportive to enumerate the design candidates to the conceptual design of a mechanical system exhaustively and automatically,but also helpful to make that enumeration process computable.展开更多
Objective To study whether free triiodothyronine （FT3） within normal range has effects on the presence and severity of coronary ar- tery disease （CAD） in different gender and age groups. Methods A total of 4206 eu...Objective To study whether free triiodothyronine （FT3） within normal range has effects on the presence and severity of coronary ar- tery disease （CAD） in different gender and age groups. Methods A total of 4206 euthyroid patients were consecutively enrolled and di- vided into CAD group （n = 3306） and non-CAD group （n = 900）. All patients underwent coronary angiography （CAG）. Gensini score （GS） was used to determine the severity of coronary artery stenosis. Severe CAD was defined as GS 〉 32 and mild CAD was defined as GS 〈 32. Logistic regression analysis and linear regression analysis were conducted to determine the association of FT3 with CAD in patients with different gender and ages. Results Concentration of FT3 was lower in patients with CAD than that in angiography-normal control group （P 〈 0.05）. In addition, concentration of FT3 was lower in severe CAD than that in mild CAD. After adjusting for traditional cardiovascular risk factors and potential confounders, FT3 was negatively correlated with the presence of CAD, but not in the old patients （〉 65 years old）. Mul- tivariable linear regression analysis showed that FT3 was negatively associated with GS in male and young patients with stable CAD, but not in the old patients. Conclusions Low FT3 within normal range was negatively associated with the presence and severity of CAD in young patients, but not in the old ones. Further studies are needed to confirm our findings.展开更多
Computational e ciency and accuracy always conflict with each other in molecular dynamics(MD) simulations. How to enhance the computational e ciency and keep accuracy at the same time is concerned by each correspondin...Computational e ciency and accuracy always conflict with each other in molecular dynamics(MD) simulations. How to enhance the computational e ciency and keep accuracy at the same time is concerned by each corresponding researcher. However, most of the current studies focus on MD algorithms, and if the scale of MD model could be reduced, the algorithms would be more meaningful. A local region molecular dynamics(LRMD) simulation method which can meet these two factors concurrently in nanoscale sliding contacts is developed in this paper. Full MD simulation is used to simulate indentation process before sliding. A criterion called contribution of displacement is presented, which is used to determine the e ective local region in the MD model after indentation. By using the local region, nanoscale sliding contact between a rigid cylindrical tip and an elastic substrate is investigated. Two two?dimensional MD models are presented, and the friction forces from LRMD simulations agree well with that from full MD simulations, which testifies the e ectiveness of the LRMD simulation method for two?dimensional cases. A three?dimensional MD model for sliding contacts is developed then to show the validity of the LRMD simulation method further. Finally, a discussion is carried out by the principles of tribology. In the discussion, two two?dimensional full MD models are used to simulate the nanoscale sliding contact problems. The results indicate that original smaller model will induce higher equivalent scratching depth, and then results in higher friction forces, which will help to explain the mechanism how the LRMD simulation method works. This method can be used to reduce the scale of MD model in large scale simulations, and it will enhance the computational e ciency without losing accuracy during the simula?tion of nanoscale sliding contacts.展开更多
Seamounts are ubiquitous topographic units in global oceans,and their influences on local oceanic circulation have attracted great attention in physical oceanography;however,previous efforts were less made in paleocli...Seamounts are ubiquitous topographic units in global oceans,and their influences on local oceanic circulation have attracted great attention in physical oceanography;however,previous efforts were less made in paleoclimatology and paleoceanography.The Caiwei Guyot in the Magellan Seamounts of the western Pacific is a typical seamount,and in this study,we investigate a well-dated sediment core by magnetic properties to reveal the relationship between deep-sea sedimentary processes and global climate changes.The principal results are as follows:(1)the dominant magnetic minerals in the sediments are low-coercivity magnetite in pseudo-single domain range,probably including a biogenic contribution;(2)the variabilities of magnetic parameters can be clustered into two sections at~500 ka,and the differences between the two units are evident in amplitudes and means;(3)changes in the grainsize-dependent magnetic parameters can be well correlated to records of global ice volume and atmospheric CO;in the middle Pleistocene.Based on these results,a close linkage was proposed between deep-sea sedimentary processes in the Caiwei Guyot and global climate changes.This linkage likely involves different roles of biogenic magnetite in the sediments between interglacial and glacial intervals,responding to changes in marine productivity and deep-sea circulation and displaying a major change in the MidBrunhes climate event.Therefore,we proposed that the sedimentary archives at the bottom of the Caiwei Guyot record some key signals of global climate changes,providing a unique window to observe interactions between various environmental systems on glacial-interglacial timescales.展开更多
Textured surfaces are widely used in engineering components as they can improve tribological properties of sliding contacts, while the detailed behaviors of nanoscale reciprocating sliding contacts of textured surface...Textured surfaces are widely used in engineering components as they can improve tribological properties of sliding contacts, while the detailed behaviors of nanoscale reciprocating sliding contacts of textured surfaces are still lack of study. By using multiscale method, two dimensional nanoscale reciprocating sliding contacts of textured surfaces are investigated. The influence of indentation depth, texture shape, texture spacing, and tip radius on the average friction forces and the running-in stages is studied. The results show that the lowest indentation depth can make all the four textured surfaces reach steady state. Surfaces with right-angled trapezoid textures on the right side are better for reducing the running-in stage, and surfaces with right-angled trapezoid textures on the left side are better to reduce wear. Compared with other textured surfaces, the total average friction forces can be reduced by 82.94%–91.49% for the case of the contact between the tip with radius R = 60rand the isosceles trapezoid textured surface. Besides,the total average friction forces increase with the tip radii due to that bigger tip will induce higher contact areas. This research proposes a detailed study on nanoscale reciprocating sliding contacts of textured surfaces, to contribute to design textured surfaces, reduce friction and wear.展开更多
Background Though type 2 diabetes mellitus(T2DM)is an important and independent risk factor for coronary artery disease(CAD)in general population,whether this feature also exists in patients with familial hypercholest...Background Though type 2 diabetes mellitus(T2DM)is an important and independent risk factor for coronary artery disease(CAD)in general population,whether this feature also exists in patients with familial hypercholesterolemia(FH)is less determined.The current study aims to characterize the clinical,laboratory,coronary and genetic characteristics of the FH patients with T2DM compared with FH alone.展开更多
In nanoscale sliding contact,adhesion effects and adhesive force are predominant,and high friction force will be produced.Friction energy is mainly converted into heat,and the heat will make nanomaterials become soft ...In nanoscale sliding contact,adhesion effects and adhesive force are predominant,and high friction force will be produced.Friction energy is mainly converted into heat,and the heat will make nanomaterials become soft to affect friction behaviors,so it is important to investigate the friction and thermal properties of the nanoscale sliding contacts.A model of a nanoscale sliding contact between a rigid cylindrical tip and an FCC copper substrate is developed by molecular dynamics simulation.The thermal properties of the substrate and the friction behaviors are studied at different sliding velocities and different tip radii.The results show that at a low sliding velocity,the friction force fluctuation is mainly caused by material melting⁃solidification,while at a high sliding velocity the material melting is a main factor for the friction reduction.The average friction forces increase at initial phase and then decrease with increasing sliding velocity,and the average temperature of the substrate increases as sliding velocity increases.Increasing tip radius significantly increases the temperature,while the coupled effects of tip radius and temperature rise make friction force increase slightly.展开更多
基金financially supported by the National Natural Science Foundation of China(Nos.U2141205,52371002,and 52374366)the Fundamental Research Funds for the Central Universities(Nos.06109125 and 06930007)Fundamental Research Funds for the Central Universities(No.FRF-BD-23-02).
文摘Laser powder bed fusion(LPBF)is a widely recognized additive manufacturing technology that can fabricate complex components rapidly through layer-by-layer formation.However,there is a paucity of research on the effect of laser scanning speed on the cellular microstructure and mechanical properties of martensitic stainless steel.This study systematically investigated the influence of laser scanning speed on the cellular microstructure and mechanical properties of a developed Fe11Cr8Ni5Co3Mo martensitic stainless steel produced by LPBF.The results show that increasing the laser scanning speed from 400 to 1000 mm/s does not lead to a noticeable change in the phase fraction,but it reduces the average size of the cellular microstructure from 0.60 to 0.35μm.The scanning speeds of 400 and 1000 mm/s both had adverse effects on performances of sample,resulting in inadequate fusion and keyhole defects respectively.The optimal scanning speed for fabricating samples was determined to be 800 mm/s,which obtained the highest room temperature tensile strength and elongation,with the ultimate tensile strength measured at(1088.3±2.0)MPa and the elongation of(16.76±0.10)%.Furthermore,the mechanism of the evolution of surface morphology,defects,and energy input were clarified,and the relationship between cellular microstructure size and mechanical properties was also established.
文摘Background Triglyceride glucose (TyG) index is a novel marker for metabolic disorders, and recently it has been reported to be associated with cardiovascular disease (CVD) risk in apparently healthy individuals.However the prognostic value of TyG index in patients with stable coronary artery disease (CAD) is not determined.
基金Supported by Key Project of National Natural Science Foundation of China(Grant No.51535009)111 Project(Grant No.B13044)
文摘Time?varying mesh stiffness(TVMS) and gear errors include short?term and long?term components are the two main internal dynamic excitations for gear transmission. The coupling relationship between the two factors is usually neglected in the traditional quasi-static and dynamic behaviors analysis of gear system. This paper investigates the influence of short?term and long?term components of manufacturing errors on quasi?static and dynamic behaviors of helical gear system considering the coupling relationship between TVMS and gear errors. The TVMS, loaded static transmission error(LSTE) and loaded composite mesh error(LCMS) are determined using an improved loaded tooth contact analysis(LTCA) model. Considering the structure of shaft, as well as the direction of power flow and bearing location, a precise generalized finite element dynamic model of helical gear system is developed, and the dynamic responses of the system are obtained by numerical integration method. The results suggest that lighter loading conditions result in smaller mesh stiffness and stronger vibration, and the corresponding resonance speeds of the system become lower. Long?term components of manufacturing errors lead to the appearance of sideband frequency components in frequency spectrum of dynamic responses. The sideband frequency components are predominant under light loading conditions. With the increase of output torque, the mesh frequency and its harmonics components tend to be enhanced relative to sideband frequency components. This study can provide effective reference for low noise design of gear transmission.
基金This work was supported,in part,by Capital Special Foundation of Clinical Application Research(No.Z121107001012015)Capital Health Development Fund(No.201614035)+1 种基金CAMS Major Collaborative Innovation Project(No.2016-I2M-1-011)PUMC Youth Fund(No.3332018200).
文摘It has been demonstrated that pitavastatin can significantly reduce low-density lipoprotein(LDL)cholesterol(LDL-C),but its impact on lipoprotein subfractions and oxidized low-density lipoprotein(oxLDL)has not been determined.The aim of the present study was to investigate the potential effects of pitavastatin on subfractions of LDL and high-density lipoprotein(HDL)as well as oxLDL in untreated patients with coronary atherosclerosis(AS).Thirty-six subjects were enrolled in this study.O f them,18 patients with AS were administered pitavastatin 2 mg/day for 8 weeks and 18 healthy subjects without therapy served as controls.The plasma lipid profile,lipoprotein subfractions and circulating oxLDL were determined at baseline and 8 weeks respectively.The results showed that pitavastatin treatment indeed not only decreased LDL-C,total cholesterol(TC),triglycerides(TG)and apolipoprotein B(ApoB)levels,and increased HDL cholesterol(HDL-C),but also reduced the cholesterol concentration of all of the LDL subfractions and the percentage of intermediate and small LDL subfractions.Meanwhile,pitavastatin could decrease plasma oxLDL levels.Furthermore,a more close correlation was found between oxLDL and LDL-C as well as LDL subfractions after pitavastatin treatment.We concluded that a moderate dose of pitavastatin therapy not only decreases LDL-C and oxLDL concentrations but also improves LDL subfractions in patients with AS.
基金National Key R&D Program of China(Grant No.2018YFB2001501)Key Program of National Natural Science Foundation of China(Grant No.51535009).
文摘Reducing the radiated noise of a gearbox is a difficult problem in aviation,navigation,machinery,and other fields.Structural improvement is the main means of noise reduction for a gearbox,and it is realized primarily through contribution analysis and structure optimization.However,these approaches have certain limitations.In this study,a low-noise design method for a gearbox that combines the two approaches is proposed,and experimental verification is performed.First,a finite element/boundary element model is established using a single-stage herringbone gearbox.Considering the vibration excitation of the gear system,the radiation noise of a single-stage gearbox is predicted based on the modal acoustic transfer vector(MATV)method.Subsequently,the maximum field point of the radiated noise is determined,and the acoustic transfer vector(ATV)analysis and modal acoustic contribution(MAC)analysis are conducted to determine the region that contributes significantly to the radiated noise of the field point.The optimization region is selected through the panel acoustic contribution(PAC)analysis.Next,to reduce the normal speed in the optimization region,topology optimization is performed.According to the topology optimization results,four different noise reduction structures are added to the gearbox,and the low-noise optimization models are established respectively.Finally,by measuring the radiated noise of the gearbox before and after optimization under a given working condition,the validity of the radiated noise prediction method and the low-noise optimization design method are verified by comparing the simulation and experimental data.A comparison of the four optimization models proves that the noise reduction effect can be achieved only by adding a noise reduction structure to the center of the density nephogram.
基金supported by the Office of Naval Research(ONR)(N00014-14-1-0533 and N00014-08-1-0642)The David and Lucille Packard Foundation
文摘This paper describes a computational study of the hydrodynamics of a ray-inspired underwater vehicle conducted concurrently with experimental measurements. High-resolution stereo-videos of the vehicle＇s fin motions during steady swimming are obtained and used as a foundation for developing a high fidelity geometrical model of the oscillatory fin. A Cartesian grid based immersed boundary solver is used to examine the flow fields produced due to these complex artificial pectoral fin kinematics. Simulations are carried out at a smaller Reynolds number in order to examine the hydrodynamic performance and understand the resultant wake topology. Results show that the vehicle＇s fins experience large spanwise inflexion of the distal part as well as moderate chordwise pitching during the oscillatory motion. Most thrust force is generated by the distal part of the fin, and it is highly correlated with the spanwise inflexion. Two sets of inter-connected vortex rings are observed in the wake right behind each fin. Those vortex rings induce strong backward flow jets which are mainly responsible for the fin thrust generation.
基金Supported by State Key Program of National Natural Science Foundation of China(Grant No.51535009)111 Project of China(Grant No.B13044).
文摘The design synthesis is the key issue in the mechanical conceptual design to generate the design candidates that meet the design requirements.This paper devotes to propose a novel and computable synthesis approach of mechanisms based on graph theory and polynomial operation.The graph framework of the synthesis approach is built firstly,and it involves:(1)the kinematic function units extracted from mechanisms;(2)the kinematic link graph that transforms the synthesis problem from mechanical domain into graph domain;(3)two graph representations,i.e.,walk representation and path representation,of design candidates;(4)a weighted matrix theorem that transforms the synthesis process into polynomial operation.Then,the formulas and algorithm to the polynomial operation are presented.Based on them,the computational flowchart to the synthesis approach is summarized.A design example is used to validate and illustrate the synthesis approach in detail.The proposed synthesis approach is not only supportive to enumerate the design candidates to the conceptual design of a mechanical system exhaustively and automatically,but also helpful to make that enumeration process computable.
文摘Objective To study whether free triiodothyronine （FT3） within normal range has effects on the presence and severity of coronary ar- tery disease （CAD） in different gender and age groups. Methods A total of 4206 euthyroid patients were consecutively enrolled and di- vided into CAD group （n = 3306） and non-CAD group （n = 900）. All patients underwent coronary angiography （CAG）. Gensini score （GS） was used to determine the severity of coronary artery stenosis. Severe CAD was defined as GS 〉 32 and mild CAD was defined as GS 〈 32. Logistic regression analysis and linear regression analysis were conducted to determine the association of FT3 with CAD in patients with different gender and ages. Results Concentration of FT3 was lower in patients with CAD than that in angiography-normal control group （P 〈 0.05）. In addition, concentration of FT3 was lower in severe CAD than that in mild CAD. After adjusting for traditional cardiovascular risk factors and potential confounders, FT3 was negatively correlated with the presence of CAD, but not in the old patients （〉 65 years old）. Mul- tivariable linear regression analysis showed that FT3 was negatively associated with GS in male and young patients with stable CAD, but not in the old patients. Conclusions Low FT3 within normal range was negatively associated with the presence and severity of CAD in young patients, but not in the old ones. Further studies are needed to confirm our findings.
基金National Natural Science Foundation of China(Grant Nos.51675429,51205313)Fundamental Research Funds for the Central Universities of China(Grant No.3102014JCS05009)111 Project of China(Grant No.B13044)
文摘Computational e ciency and accuracy always conflict with each other in molecular dynamics(MD) simulations. How to enhance the computational e ciency and keep accuracy at the same time is concerned by each corresponding researcher. However, most of the current studies focus on MD algorithms, and if the scale of MD model could be reduced, the algorithms would be more meaningful. A local region molecular dynamics(LRMD) simulation method which can meet these two factors concurrently in nanoscale sliding contacts is developed in this paper. Full MD simulation is used to simulate indentation process before sliding. A criterion called contribution of displacement is presented, which is used to determine the e ective local region in the MD model after indentation. By using the local region, nanoscale sliding contact between a rigid cylindrical tip and an elastic substrate is investigated. Two two?dimensional MD models are presented, and the friction forces from LRMD simulations agree well with that from full MD simulations, which testifies the e ectiveness of the LRMD simulation method for two?dimensional cases. A three?dimensional MD model for sliding contacts is developed then to show the validity of the LRMD simulation method further. Finally, a discussion is carried out by the principles of tribology. In the discussion, two two?dimensional full MD models are used to simulate the nanoscale sliding contact problems. The results indicate that original smaller model will induce higher equivalent scratching depth, and then results in higher friction forces, which will help to explain the mechanism how the LRMD simulation method works. This method can be used to reduce the scale of MD model in large scale simulations, and it will enhance the computational e ciency without losing accuracy during the simula?tion of nanoscale sliding contacts.
基金The Natural Science Foundation of Shanghai under contract No.19ZR1459800the Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou)under contract No.GML2019ZD0106the Project of Global Changing and Air-sea Interaction under contract No.GASI-GEOGE-04
文摘Seamounts are ubiquitous topographic units in global oceans,and their influences on local oceanic circulation have attracted great attention in physical oceanography;however,previous efforts were less made in paleoclimatology and paleoceanography.The Caiwei Guyot in the Magellan Seamounts of the western Pacific is a typical seamount,and in this study,we investigate a well-dated sediment core by magnetic properties to reveal the relationship between deep-sea sedimentary processes and global climate changes.The principal results are as follows:(1)the dominant magnetic minerals in the sediments are low-coercivity magnetite in pseudo-single domain range,probably including a biogenic contribution;(2)the variabilities of magnetic parameters can be clustered into two sections at~500 ka,and the differences between the two units are evident in amplitudes and means;(3)changes in the grainsize-dependent magnetic parameters can be well correlated to records of global ice volume and atmospheric CO;in the middle Pleistocene.Based on these results,a close linkage was proposed between deep-sea sedimentary processes in the Caiwei Guyot and global climate changes.This linkage likely involves different roles of biogenic magnetite in the sediments between interglacial and glacial intervals,responding to changes in marine productivity and deep-sea circulation and displaying a major change in the MidBrunhes climate event.Therefore,we proposed that the sedimentary archives at the bottom of the Caiwei Guyot record some key signals of global climate changes,providing a unique window to observe interactions between various environmental systems on glacial-interglacial timescales.
基金Supported by National Natural Science Foundation of China(Grant Nos.51675429,51205313)Fundamental Research Funds for the Central Universities,China(Grant No.3102014JCS05009)the 111 Project,China(Grant No.B13044)
文摘Textured surfaces are widely used in engineering components as they can improve tribological properties of sliding contacts, while the detailed behaviors of nanoscale reciprocating sliding contacts of textured surfaces are still lack of study. By using multiscale method, two dimensional nanoscale reciprocating sliding contacts of textured surfaces are investigated. The influence of indentation depth, texture shape, texture spacing, and tip radius on the average friction forces and the running-in stages is studied. The results show that the lowest indentation depth can make all the four textured surfaces reach steady state. Surfaces with right-angled trapezoid textures on the right side are better for reducing the running-in stage, and surfaces with right-angled trapezoid textures on the left side are better to reduce wear. Compared with other textured surfaces, the total average friction forces can be reduced by 82.94%–91.49% for the case of the contact between the tip with radius R = 60rand the isosceles trapezoid textured surface. Besides,the total average friction forces increase with the tip radii due to that bigger tip will induce higher contact areas. This research proposes a detailed study on nanoscale reciprocating sliding contacts of textured surfaces, to contribute to design textured surfaces, reduce friction and wear.
文摘Background Though type 2 diabetes mellitus(T2DM)is an important and independent risk factor for coronary artery disease(CAD)in general population,whether this feature also exists in patients with familial hypercholesterolemia(FH)is less determined.The current study aims to characterize the clinical,laboratory,coronary and genetic characteristics of the FH patients with T2DM compared with FH alone.
基金Sponsored by the National Natural Science Foundation of China(Grant Nos.52075444,51675429)the Key Project of National Natural Science Foundation of China(Grant No.51535009)the Fundamental Research Funds for the Central Universities(Grant No.31020190503004).
文摘In nanoscale sliding contact,adhesion effects and adhesive force are predominant,and high friction force will be produced.Friction energy is mainly converted into heat,and the heat will make nanomaterials become soft to affect friction behaviors,so it is important to investigate the friction and thermal properties of the nanoscale sliding contacts.A model of a nanoscale sliding contact between a rigid cylindrical tip and an FCC copper substrate is developed by molecular dynamics simulation.The thermal properties of the substrate and the friction behaviors are studied at different sliding velocities and different tip radii.The results show that at a low sliding velocity,the friction force fluctuation is mainly caused by material melting⁃solidification,while at a high sliding velocity the material melting is a main factor for the friction reduction.The average friction forces increase at initial phase and then decrease with increasing sliding velocity,and the average temperature of the substrate increases as sliding velocity increases.Increasing tip radius significantly increases the temperature,while the coupled effects of tip radius and temperature rise make friction force increase slightly.
